Handbook for Sound Engineers

314 Chapter 12

Output capacitance is measured from the plate to all
other elements, except the control grid, which is
connected to ground.
Grid-to-plate capacitance is measured from the
control grid to the plate with all other elements
connected to ground.

12.1.8 Plate Resistance

The plate resistance (rp) of a vacuum tube is a constant
and denotes the internal resistance of the tube or the
opposition offered to the passage of electrons from the
cathode to the plate. Plate resistance may be expressed
in two ways: the dc resistance and the ac resistance. Dc
resistance is the internal opposition to the current flow
when steady values of voltage are applied to the tube
elements and may be determined simply by using
Ohm’s Law

(12-5)

where,
Ep is the dc plate voltage,
Ip is the steady value of plate current.

The ac resistance requires a family of plate-current
curves from which the information may be extracted.
As a rule, this information is included with the tube
characteristics and is used when calculating or selecting
components for an amplifier. The equation for calcu-
lating ac plate resistance is

(12-6)

where,
' Ep is the change in voltage at the plate,
' Ip is the change in plate current,
Esig is the control grid signal voltage and is held
constant.

The values of Ep and Ip are taken from the family of
curves supplied by the manufacturer for the particular
tube under consideration.

12.1.9 Grid Bias

Increasing the plate voltage or decreasing the grid-bias
voltage decreases the plate resistance. The six methods
most commonly used to bias a tube are illustrated in
Fig. 12-5. In Fig. 12-5A bias cell (battery) is connected
in series with the control grid. In Fig. 12-5B the tube is

self-biased by the use of a resistor connected in the

cathode circuit. In Fig. 12-5C the circuit is also a form

of self-bias; however, the bias voltage is obtained by the

use of a grid capacitor and grid-leak resistor connected

between the control grid and ground. In Fig. 12-5D the

bias voltage is developed by a grid-leak resistor and

capacitor in parallel, connected in series with the

control grid. The method illustrated in Fig. 12-5E is

called combination bias and consists of self-bias and

battery bias. The resultant bias voltage is the negative

voltage of the battery, and the bias created by the

self-bias resistor in the cathode circuit. Another combi-

nation bias circuit is shown in Fig. 12-5F. The bias

battery is connected in series with the grid-leak resistor.

The bias voltage at the control grid is that developed by

the battery and the self-bias created by the combination

of the grid resistor and capacitor.

If the control grid becomes positive with respect to

the cathode, it results in a flow of current between the

rp

dc

Ep

Ip

----- -=

rpac

'Ep

'Ip

----------=

Figure 12-5. Various methods of obtaining grid bias.

Bias

cell

A. Fixed-bias battery. B. Self-bias.

Rg

Rg

Rk Ck

+

Cg

Cg

Rg

Rg

Input

C. Grid-leak bias. D. Grid-leak bias.

Rg

Cg

Rk Ck

Rg

Input

E. Combination bias. F. Combination bias.

+

Bias

cell

Bias

cell

Input

Input Input

Input